1. Field of the Invention
This invention relates to a wellhead severing system for removing wellheads from the sea floor and, in particular, to an explosive severing system which compensates for variations in pressure and temperature to maintain a constant volume of liquid explosive in the charge container.
2. Description of the Prior Art
With the increased exploration for petroleum products beneath ocean floors, nationalities have enacted strict requirements to govern such operations. One such requirement is the removal of wellheads and a specified depth of casing upon abandonment of a well. Various removal systems have been developed including mechanical severing systems which cut the casing to facilitate removal of the wellhead and explosive severing systems which rupture a section of casing below the ocean floor to allow retrieval of the wellhead.
The state of the art in marine wellhead removal involves using a liquid explosive, usually sensitized nitromethane, poured into a cylindrical charge container which is lowered into the marine wellhead and casing on the sea floor. The liquid explosive is then detonated to sever the casing allowing the wellhead and other attached structure to be pulled from the sea floor. However, it has been learned that when liquid nitromethane is lowered through the several thousand feet of ocean, the combined effects of external pressure as a result of water depth and decreased ambient temperature causes the liquid charge to contract or decrease in volume. This contraction can decrease the strength of the explosive blast and possibly result in a failure to sever the casing. Historically, the liquid contraction was avoided by leaving slight openings in the charge containers allowing sea water to fill the void created by the contracting liquid charge. In the case of nitromethane, which is heavier than water, the liquid explosive remained in the container as a result of gravity. Nevertheless, prolonged exposure of the liquid charge to sea water adversely affects the strength of the explosive or can result in a complete dilution of the charge.